Quite a bit stronger, at least in the canon.
The Spartan-II project was about individual combat effectiveness, with little care of cost. At the time of the Spartan-II project, the Insurrectionist threat had kicked up dramatically, and the human empire was in danger of falling apart. It was just a coincidence that the Covenant found us around the same time. The Spartan-III project was about producing a massive number of Spartans to throw against the Covenant, while keeping the cost down, so that the money could be spent evenly for other vital projects. These Spartans were less effective because of cheaper armor, lesser training, and few biological augments. So, the Spartan-II armor would be stronger, but it is not recognizable in-game.